We wish to continue our work on the purification, characterization and mechanism studies of human erythropoietin (Ep). Our main objectives are: (1) To prepare sufficient quantities of highly purified and well characterized Ep. (2) To study the mechanism of Ep action during stem cell differentiation. A simple two-step procedure for Ep purification has been designed and developed in our laboratory. It involves the use of hydrophobic interaction chromatography and double immunoaffinity chromatography. These are new and unconventional techniques. They are effective and rapid. These methods would be used for our Ep purification. Two molecular species of Ep, Ep alpha and Ep beta have been isolated and extensively purified. They showed similar specific activity (20,000 micron/mg of protein) but different molecular weights, 56,000 and 38,000 daltons for Ep alpha and Ep beta, respectively. Their other physiochemical properties are being characterized. The molecular mechanism of their actions would be studied in normal erythroid, and in erythroleukemic cell lines.